The present invention relates to transparent glass ceramics, in particular, to substantially transparent glass-ceramics based on orthosilicate crystals of various polymorphs within the ternary Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system.
Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. The method for producing such glass-ceramics customarily involves three fundamental steps: first, a glass-forming batch is melted; second, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and third, the glass body is heated to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ.
Frequently, the glass body is exposed to a two-stage treatment. Hence, the glass will be heated initially to a temperature within, or somewhat above, the transformation range for a period of time sufficient to cause the development of nuclei in the glass. Thereafter, the temperature will be raised to levels approaching, or even exceeding, the softening point of the glass to cause the growth of crystals on the previously-formed nuclei. The resultant crystals are commonly more uniformly fine-grained and the articles are typically more highly crystalline. Internal nucleation allows glass-ceramics to possess such favorable qualities as a very narrow, particle size distribution and highly uniform dispersion throughout the glass host.
Transparent glass-ceramics are well known to the art; the classic study thereof being authored by G. H. Beall and D. A. Duke in xe2x80x9cTransparent Glass-Ceramicsxe2x80x9d, Journal of Materials Science, 4, pp. 340-352 (1969). Glass-ceramic bodies will display transparency to the human eye when the crystals present therein are considerably smaller than the wavelength of visible light. More specifically, transparency generally results from crystals less than 50 nm, and preferably as low as 10 nm in size.
Recently, much effort has been concentrated in the area of using transparent glass-ceramics as hosts for transition metals which act as optically active dopants. Suitable glass-ceramic hosts must be tailored such that transition elements will preferentially partition into the crystals. Co-pending Provisional Application Serial No. 60/160,053 entitled xe2x80x9cTransition Metal Glass-Ceramicsxe2x80x9d by Beall et al., co-assigned to the present assignee, and herein incorporated by reference in its entirety, is directed to transition-metal doped glass-ceramics suitable for formation of a telecommunications gain or pump laser fiber.
Transparent glass-ceramics which contain relatively small numbers of crystals can be of great use in cases where the parent glass provides an easy-to-melt or an-easy-to-form vehicle for a crystal. The crystal in itself, may be difficult or expensive to synthesize, but provides highly desirable features, such as optical activity. The crystals in the glass-ceramic are generally oriented randomly throughout the bulk of the glass, unlike a single crystal which has a specific orientation. Random orientation, and consequent anisotropy, are advantageous for many applications. One example is that of optical amplifiers, where polarization-independent gain is imperative.
Transparent glass-ceramics doped with transition elements can combine the optical efficiency of crystals with the forming flexibility of glass. For example, both bulk (planar) and fiber forms can be fabricated from these glass-ceramics.
Therefore, there exists a need for transparent, glass-ceramic materials which contain small tetrahedral and interstitial sites, and hence are suitable as potentially valuable hosts for small, optically active transition elements. Such elements include, but are not limited to, Cr4+, Cr3+, Co2+, Cu2+, Ni2+, Mn2+, Fe3+, Fe2+, and Cu+. These elements impart luminescence and fluorescence thereto, yielding doped, glass-ceramic materials suitable for application in the optical field industry.
Numerous crystal phases, comprising multiple polymorphs and wide ranges of solid solution, occur within the composition field defined by Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4. These crystal phases have unique crystal structures that are distinct from the crystal structures of the end members Mg2SiO4 (Forsterite), Zn2SiO4 (Willemite), and Li4SiO4. The crystal structures are based on hexagonal close-packed arrays of oxygen atoms, with all the metal ions (i.e., Zn2+, Mg2+, and Li+) in tetrahedral coordination. Different distributions of cations over the tetrahedral sites give rise to numerous polymorphs which in turn are classified as xcex2- and xcex3-type structures. The linkage of the tetrahedra in the xcex2-type structures is such that each tetrahedron shares each of its corners with three other tetrahedra, whereas in the closely-related xcex3-type structures some tetrahedral edge sharing occurs as well. The crystal structures are not well characterized, but they display orthorhombic or pseudo-orthorhombic (monoclinic) symmetry.
While many of these phases and their stability fields have been described in phase equilibria and mineralogical literature, no transparent glass-ceramics based on crystals within the ternary Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system have ever been fabricated.
Accordingly, the primary object of the present invention is to provide glass-ceramic materials which are substantially and desirably totally transparent, and which contain a predominant, orthosilicate crystal phase whose composition lies within the ternary Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system.
Another object of the present invention is to provide such orthosilicate glass-ceramics that are capable of being doped with ingredients which confer luminescence and/or fluorescence thereto.
An important advantage of the present glass-ceramic family is that it provides a material containing orthosilicate crystals which contain tetrahedrally-coordinated sites into which there is partitioning of transition metal ions including, but not limited to, Cr4+, Cr3+, Co2+, Cu2+, Ni2+, Mn2+, Fe3+, Fe2+, and Cu+. The material is glass-based thus providing an important flexibility that allows for fabrication of both bulk (such as planar substrates) and fiber (such as optical fiber) forms.
Other objects and advantages of the present invention will be apparent from the following description.
In accordance with the present invention, there is provided a, substantially transparent, orthosilicate glass-ceramic whose crystal composition lies within the ternary Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system. The inventive glass-ceramic is formed from precursor glasses having the following compositions, in weight percent on an oxide basis: 35-72 SiO2, 0-25 Al2O3, 0-40 ZnO, 0-18 MgO, 1-15 Li2O, 0-18 K2O, 0-8 Na2O, 0-8 P2O5, with the condition that xcexa3ZnO+MgOxe2x89xa77.
To obtain the greatest degree of transparency in the final glass-ceramic article, the most preferred compositions in the Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system will consist essentially, expressed in terms of weight percent on the oxide basis, of 40-65 SiO2, 5-18 Al2O3, 8-36 ZnO, 0-12 MgO, 1-12 Li2O, 8-14 K2O, 0-5 Na2O, 0-6 P2O5, with the condition that xcexa3ZnO+MgOxe2x89xa710.
One method of obtaining optical activity in the present inventive orthosilicate glass-ceramic materials, i.e., fluorescence, over the communications transmission wavelength range of 1100 to 1700 nm, is to dope the glass-ceramic material with up to about 1 wt. % chromium oxide and preferably with about 0.003-0.3 wt. %.
A method of making the inventive glass-ceramic is also provided comprising the steps of:
a.) melting a batch for a glass having a composition consisting essentially, in weight percent on an oxide basis, of 35-72 SiO2, 0-25 Al2O3, 0-40 ZnO, 0-18 MgO, 1-15 Li2O, 0-18 K2O, 0-8 Na2O, 0-8 P2O5, with the condition that xcexa3ZnO+MgOxe2x89xa77;
b.) cooling the glass to a temperature at least below the transformation range of the glass;
c.) exposing the glass to a temperature between about 550-950xc2x0 C. for a period of time sufficient to cause the generation of a glass-ceramic which is substantially transparent and which contains a predominant, orthosilicate crystal phase within the ternary Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system; and,
d.) cooling the glass-ceramic to room temperature.